composition.md

From Chapter 21:

Explaining the double . in:

(sequence .) . fmap

The weird looking composition, which you’ve possibly also seen in the form of (join .) . fmap is because fmap takes two (not one) arguments, so the expressions aren’t proper unless we compose twice to await a second argument for fmap to get applied to.

(sequence .) . fmap ≣ \ f xs -> sequence (fmap f xs)

whereas:

sequence . fmap ≣ \ f -> sequence (fmap f)